It is well known that the number of colon cancer cases has recently been remarkably increasing in Japan. This rise is partly attributable to the adoption of Western-style dietary habits.
It is highly important to discover colon cancer at an early stage from the viewpoints of both prevention and treatment. Known methods for the early detection of colon cancer include use of tumor markers in blood, detection of human blood in feces and image diagnosis. Among these methods, detection of human blood in feces may be cited as an inexpensive test method which can be carried out with little burden on a patient [Yakuri to Chiryo, Vol. 16, No. 3, March (1988) pp. 173-181; and Gan no Rinsho, Vol. 33, No. 9, August (1987) pp. 48-61]. However, this method, which comprises detecting hemoglobin due to hemorrhage from a cancer tissue by using a monoclonal antibody, cannot be regarded as an excellent method, since it suffers from some problems in, for example, distinguishing hemorrhage caused by some factor other than cancer and a change in hemoglobin with the lapse of time.
Regarding a casual relationship between bile acids and the incidence of colon cancer, it has been pointed out that bile acids, in particular, secondary bile acids act as a promoter in the pathogenesis colon cancer (i.e., a so-called auxiliary substance in oncogenesis) by immunological studies and animal experiments both in Japan and abroad [Igaku no Ayumi, Vol. 147, No. 5 (1988) pp. 395-398; Mutation Research, 238 (1990) pp. 313-320; Medical Practice, Vol. 8, No. 6 (1991) pp. 879-881; and Hill. M. J.: The role of unsaturated bile acids in the etiology of large bowel cancer, in Origins of Human Cancer., ed. by Watson. J. D. and Winstein. J. A., Cold Spring Habor Conference on cell proliferation, Vol. 4. Cold Spring Habor Laboratory, New York pp. 1627-1640, (1977); and Oncology 40: pp. 255-258 (1983)]. In a pilot test performed by the present inventors, a large amount of secondary bile acids were detected in colon cancer patients.
On the other hand, bile acids produced in the liver are discharged into the gallbladder in the form of the primary bile acids (cholic acid: CA) and concentrated therein. Next, they are released to the bowels by meal stimulation. Then they emulsify fats and lipids and thus facilitate the absorption of these substances from the intestinal wall. Most bile acids per se repeat the closed enterohepatic circulation where they are reabsorbed mainly in the ileum and returned to the liver through the portal vein. However, some bile acids are discharged in the feces and are reduced into their secondary bile acids (deoxycholic acid: DCA) by enterobacteria. Accordingly, fecal bile acids are mostly free-type secondary bile acids [Tanida N, Hisaka Y, Hosomi M. et al.: Fecal bile acid analysis in healthy japanese subjects using a lipophilic anion exchanger, capillary column gas chromatography and mass spectrometry. Gastroenterologia jpn 16: 363, 1981; and Wakayama Igaku, (1986), 37 (3) pp. 195-202].
In clinical practice, it is known that the amount and composition of fecal bile acids are changed in various diseases in digestive tracts or after the excision of digestive tracts [Igaku no Ayumi, Vol. 149. No. 13 (1989) pp. 953-954; Medical Practice, Vol. 8, No. 6 (1991) pp. 879-881].
Analysis of fecal bile acids is important in the clarification of the metabolic mechanism of bile acids. Conventional methods for analyzing these bile acids include GLC (gas-liquid chromatography) and GC-MS (gas-liquid chromatography-mass spectrometry). Further, there have been known a simplified method for measuring bile acids in human feces with the use of hydroxysteroid dehydrogenase (3.alpha.-HSD, 7.alpha.-HSD) for mass assay and an enzyme fluorescent method as its improved method.
However, the above-mentioned GLC and GC-MS require complicated analytical procedures and a high degree of knowledge and technique, which makes them less applicable to mass assay.
On the other hand, the latter enzyme fluorescent method is disadvantageous since it still suffers from the problem of .beta.-hydroxybile acid in feces and requires the preparation of freeze-dried feces after the collection and extraction and purification of bile acids. Thus this method is less applicable to mass assay too. Moreover, these tests are designed to measure only the total bile acids. Namely, neither the respective contents of the primary and secondary bile acids nor the composition can be clarified thereby. Thus the enzyme fluorescent method is less useful in screening colon cancer.
The present invention has been completed in order to solve the above-mentioned problems. Thus it aims at providing anti-bile acid antibodies, which can be effectively used as an immunological assay reagent for measuring and examining primary and secondary bile acids in feces to thereby make it possible to detect colon cancer at an early stage and to identify latent colon cancer, and a method for assaying bile acids in feces by using these antibodies.